1. Field of the Invention
The present invention relates to a wrench with a fixed maximum operational torque to prevent damage to the object secured by a fastener driven by the wrench.
2. Description of the Related Art
FIG. 10A of the drawings illustrates a conventional wrench 1 having a hexagonal driving portion with six planar faces for engaging with six faces of a hexagonal groove in a top face of a fastener. However, slide tends to occur between the planar faces of the driving portion of the wrench 1 and the faces of the fastener. FIG. 10B illustrates a so-called TORX wrench 3 having plural arcuate faces for engaging with corresponding arcuate faces in a top face of a fastener. Such a TORX wrench 3 is used to tighten important parts of a car and cutting tools. As illustrated in FIG. 11, a blade 7 is tightened to a cutting tool 5 by a bolt 6. However, it was found that the expensive blade 7 tends to be damaged when the bolt 6 is excessively tightened. However, the blade 7 could fly away and thus cause injury if the bolt 6 is not tightened to the desired extent.
An object of the present invention is to provide a wrench with a fixed maximum operational torque such that when the torque applied by the user is greater than the maximum operational torque, the wrench slides and the fastener is not turned. Thus, damage to the object secured by the fastener is prevented.
Another object of the present invention is to provide a wrench with a fixed maximum operational torque that can be altered in response to the actual use.
A wrench comprises a rod comprising a driving portion for engaging with a fastener, a retainer having an end securely engaged with the rod to move therewith, and a casing comprising a compartment for accommodating the retainer. The casing comprises a retaining section defining a retaining space for retaining the other end of the retainer in place. When a rotational force applied to the casing is smaller than an engaging force between the retaining section of the casing and a retaining device that is attached between the retaining section and the retainer, the retainer and the rod are turned to thereby turn the fastener. When a rotational force applied to the casing is greater than the engaging force, the casing slides while the retainer and the rod are not turned.